1. Field of the Invention
This invention is directed to a method for improving the stability of aqueous solutions of cationic thermosetting polyamide-epichlorohydrin resins. The present invention is directed specifically to a method for improving the stability of an aqueous solution of a cationic thermosetting polyamide-epichlorohydrin resin by acidifying the resin with an aqueous mixture of a weak acid and a strong acid.
2. Description of Related Art
Cationic thermosetting resins prepared by reacting a polyamide of a polyalkylene polyamine such as diethylenetriamine, and a dicarboxylic acid such as adipic acid, with epichlorohydrin are known to be efficient wet-strength resins for paper. For example, see Keim U.S. Pat. No. 2,926,154. Such resins can be used under alkaline, neutral or acidic conditions.
One consequence of the high activity exhibited by these resins is that safeguards must be observed to prevent premature gellation of the resin and loss of wet-strength efficiency. It is known that limiting the solids content of aqueous solutions of such polyamide-epichlorohydrin resins to below about 5 to 10 wt.% and adjusting the pH of the aqueous resin solution to below about 3.0 using a strong mineral acid, such as sulfuric acid, stabilizes the aqueous resin against gellation for relatively long time periods. However, it also is known that as the solids content of the aqueous resin solution increases, its storage stability is degraded significantly.
A variety of procedures are available which purportedly stabilize high solids content aqueous solutions of such resins. Keim U.S. Pat. No. 3,240,761, for example, includes a quaternizing agent such as an alkyl halide during the latter stages of the polyamide-epichlorohydrin reaction. Coscia U.S. Pat. No. 3,259,600 describes adding a stoichiometric excess of certain metal complexing salts to the aqueous resin solution in order to form metal coordination complexes which purportedly enhance resin stability. Earle U.S. Pat. No. 3,352,833 describes using an acidic hydrogen halide such as hydrochloric acid, to stabilize the epichlorohydrin moiety of such aqueous resins without reducing wet strength efficiency by forming the corresponding aminochlorohydrin hydrochloride.
Schmalz U.S. Pat. No. 3,197,427 indicates that the storage stability of certain cationic polyamide-epichlorohydrin resins can be improved by adjusting the pH of the aqueous resin first with formic acid and then with sulfuric acid. The formic acid is added to the aqueous resin in an amount sufficient to adjust its pH into the range of about 3 to 4. Afterward, sulfuric acid is added in an amount sufficient to reduce the pH of the aqueous resin further into the range of about 2 to 3. It is asserted that this procedure provides an aqueous resin system with good stability against gelation and loss of wet-strength efficiency over relatively long periods of time at solids contents of 25% and above.
The prior art has concerned itself primarily with stabilizing such aqueous solutions of polyamide-epichlorohydrin resins against gellation, and has not considered changes in the resin's viscosity which might occur during storage. A common consequence of acid stabilization using a strong mineral acid is an initial degradation of the resin's viscosity due to acid hydrolysis of the polyamide-epichlorohydrin polymer. Even though the viscosity may on some occasions subsequently increase to a desired level due to resin crosslinking, the initial viscosity degradation in the aqueous resin solution has been associated with a loss in the wet-strength efficiency of the resin. For example, although the variation of resin viscosity with time is not reported, data reported in Schmalz, see particularly Examples 10, 11, and 12 (Table 2), indicate that while resin gelation is avoided by the acid treatment, substantial resin hydrolysis does occur during storage as evidenced by the significant viscosity degradation of the stabilized resins.
A procedure which stabilizes high solids content aqueous solutions of such polyamide-epichlorohydrin resins against gellation while at the same time providing stability against a significant loss in solution viscosity therefore would constitute a significant improvement. The present invention has an its objective that result.
It has been discovered that the storage stability of high solids content, aqueous cationic thermosetting polyamide-epichlorohydrin resins useful for imparting wet-strength to paper can be improved by using an aqueous mixture of a weak acid such as formic acid and a strong acid such as sulfuric acid as the acidification agent.